Add FastAPI backend for energy trading system

Implements FastAPI backend with ML model support for energy trading,
including price prediction models and RL-based battery trading policy.
Features dashboard, trading, backtest, and settings API routes with
WebSocket support for real-time updates.

backend/tests/conftest.py

@@ -0,0 +1,32 @@
+import pytest
+from fastapi.testclient import TestClient
+from app.main import app
+
+
+@pytest.fixture
+def client():
+    return TestClient(app)
+
+
+@pytest.fixture
+def sample_price_data():
+    return {
+        "timestamp": "2024-01-01T00:00:00",
+        "region": "FR",
+        "day_ahead_price": 50.0,
+        "real_time_price": 55.0,
+        "volume_mw": 1000.0,
+    }
+
+
+@pytest.fixture
+def sample_battery_state():
+    return {
+        "timestamp": "2024-01-01T00:00:00",
+        "battery_id": "battery_1",
+        "capacity_mwh": 100.0,
+        "charge_level_mwh": 50.0,
+        "charge_rate_mw": 50.0,
+        "discharge_rate_mw": 50.0,
+        "efficiency": 0.9,
+    }